Blender: When resizing the body of the model, the eyes are not affected
Your armature can have a  bone to the eyes so that when the armature is scaled so are the eyes.  Create one or two eyeball bones which are connected to the head bone or neck bone.  Thus when you scale armature master bone to scale the fish body, the eyeball bone will scale too.

These are typical settings for the eyeball bone.

Verify the inherit rotation and inherit scale settings.





You can have the eyes use a single bone of the armature as [Bone] (object) parent.  So that means no weight painting is necessary.  Thus the bone is behaving like an [empty] in the relationship of being a parent to the eyes.